Possible mechanisms of TSH--independent thyroid growth

Photoperiodic modulation of ovarian metabolic, survival, proliferation and gap junction markers in adult golden hamster, Mesocricetus auratus

Female reproductive physiology is greatly dependent on tight regulation of metabolic and survival factors. Photoperiod regulates female reproductive rhythms but very less information exists explaining whether photoperiod could modulate thyroid hormone homeostasis, metabolic/energy parameters along with survival, proliferation and gap junction proteins in the ovary of a long-day breeder, Mesocricetus auratus. Adult female hamsters were exposed to different photoperiodic regimes i.e., critical photoperiod (CP; 12.5L:11.5D), short photoperiod (SP; 8L:16D) and long photoperiod (LP; 16L:8D) for 12 weeks. LP upregulated thyroidal and gonadal activity as apparent by histoarchitecture, thyroid hormone profile [triiodothyronine (T3), thyroxin (T4) and thyroid stimulating hormone (TSH)], luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol (E2) and progesterone (P4) levels when compared with SP exposed hamsters. Further, LP increased thyroid hormone receptor-α/deiodinase-2 (TRα/Dio-2), estrogen receptor-α (ERα)/aromatase and insulin receptor/glucose transporter-4 (IR/GLUT-4) expressions in ovary. Interestingly, ovarian sirtuin-1 (SIRT-1) expression was also upregulated under LP condition along with cell proliferation (proliferating cell nuclear antigen or PCNA), survival (B cell lymphoma-2 or Bcl-2) and gap junction (connexin-43) markers when compared to SP exposed hamsters. We also noted elevated levels of circulatory leptin, insulin along with melatonin and its receptor (MT-1) in ovary under SP condition. Thus, we suggest that photoperiod plays a vital role in regulation of thyroid and reproductive hormone homeostasis along with key metabolic and survival markers in the ovary of adult golden hamsters, M. auratus providing further insight into the regulation of female reproductive seasonality in a long-day breeder.

Photoperiodic response of pineal-thyroid axis of the female Indian palm squirrel, Funambulus pennanti

Effect of short (SP: 10 L: 14 D: Light: Dark) and long (LP: 14 L: 10 D) photoperiod was tested on pineal-thyroid axis in a tropical seasonally breeding rodent F. pennanti during sexually regressive phase of reproduction. SP reduced thyroid weight and plasma thyroxine (T4) as well as increased melatonin (aMT) while pinealectomy (Px) prevented such an effect of SP. LP and Px plus LP had no significant effect on thyroid activity when compared with their respective controls. These results suggest that SP stimulated pineal activity which increased the concentration of aMT, hence inhibited thyroid gland activity. However, the plasma aMT concentration under LP regime had no significant difference from controls, as the ambient photoperiodic length (13.6 hr) was almost similar to the experimental one (14.0 hr). It is suggested that being innervated from the same ganglion (SCG) which conveys photic stimuli to the pineal gland, the thyroid gland like the pineal is also under the influence of photoperiodic responses. Further pineal/melatonin also influences thyroid function indirectly.

Effects of melatonin on [3H]-thymidine incorporation into DNA of rat thyroid lobes in vitro

The effects of 4-h incubation in the presence of melatonin and N-acetylserotonin on the incorporation of [3H]-thymidine into DNA of the rat thyroid lobes were investigated. Additionally, thyroid lobes were incubated in the exposure to melatonin with thyrotropin jointly. Melatonin, when applied in the lowest concentration examined (10(-9)M), inhibited [3H]-thymidine incorporation, the effect of other concentrations (10(-6)M-10(-8)M) being not significant. N-acetylserotonin (10(-6)M) did not affect the rate of incorporation of that labelled nucleoside. As expected, thyrotropin enhanced the [3H]-thymidine incorporation into DNA of thyroid lobes. Melatonin (10(-7)M) suppressed that stimulatory effect of thyrotropin. The participation of melatonin in the regulation of thyroid growth processes is considered.

Influence of melatonin and N-acetylserotonin on the cyclic AMP concentration in the rat thyroid lobes incubated in vitro

Effects of melatonin and N-acetylserotonin on the cyclic AMP (cAMP) concentration in the organ-cultured rat thyroid gland were investigated. Exposure of the thyroid explants to melatonin (10(-8) M) resulted in a decrease of cAMP levels. Melatonin at higher concentrations (10(-7) M and/or 10(-6) M) failed to influence the thyroid cAMP level. N-acetylserotonin (10(-6) M), like melatonin in the lowest concentration employed, reduced cAMP concentrations in the thyroid explants when compared with controls. Unexpectedly, melatonin (at a concentration of 10(-7) M) added to the incubation medium with TSH (60 mU/ml), decreased the cAMP concentration in the thyroid compared with the group exposed to melatonin (10(-7) M) alone.

Circannual oscillations of thyroxine and cholesterol levels after pinealectomy and ganglionectomy related to the weight changes of the pineal, pituitary, and thyroid glands

Circannual rhythms of thyroxine (T4) and cholesterol blood levels, as well as of pineal, pituitary and thyroid weights were compared in intact, pinealectomized (Px), and ganglionectomized (Gx) rats. Special emphasis was put on the similarities and differences between the effect of Px and of Gx exerted on the neuroendocrine regulation of thyroid function. Animals were kept under changing lighting schedules simulating the natural seasonal conditions (December, L:D 8:16; June, L:D 16:8; March and September, L:D 12:12). Animals were killed 90 d after surgical interventions in the above-mentioned 4 months. In each season, animals were killed at four time points of the daily rhythm, and the means of the four data of a single day were used for further analysis evaluating circannual changes. All the investigated five parameters showed a monophasic circannual rhythm. The lowest T4 blood level was measured in summer; in contrast, cholesterol level and organ weights had a maximum in the same season of the year. The annual mean value of T4 level was diminished, while cholesterol level and pituitary and thyroid weights were increased in Px and Gx animals. Pineal weight showed a decrease in the Gx animals. Both Px and Gx failed to influence the circannual rhythms of the measured parameters. However, a significant T4-decreasing effect of Px and Gx was observed in the second half of the year, whereas a cholesterol-level-increasing effect, as well as thyroid hypertrophy, was registered in the first half of the year. These results suggest a prothyroid function of the pineal gland.

Melatonin inhibits the basal and TSH-stimulated mitotic activity of thyroid follicular cells in vivo and in organ culture

The aim of the present study has been to examine the effect of melatonin, administered to mice as daily subcutaneous injections for a total of 10 days, on the mitotic activity of thyroid follicular cells (TFC). The colchicine metaphase-arrest technique was employed in the experiment. We found that melatonin (10 micrograms and/or 100 micrograms injection) decreased significantly the mean mitotic activity rate (MMAR) of TFC in both male and female mice. Moreover, melatonin totally suppressed the stimulatory effect of TSH on the MMAR of TFC in both sexes of mice. Furthermore, the effect of melatonin (5 X 10(-7) M) on the proliferation of TFC in the organ-cultured rat and mouse thyroid explants was investigated. It was found that melatonin almost totally suppressed the MMAR of TFC in organ culture. Moreover, melatonin blocked the stimulatory effect of TSH on the MMAR of TFC in both rat and mouse thyroid explants. N-acetylserotonin (NAc-5HT, 10(-6) M) also decreased the MMAR of cultured thyroid explants, but its effect was less expressed when compared to melatonin inhibition. The present data indicate that melatonin can exert its inhibitory effect on the proliferation of TFC directly at the thyroid level, since this pineal indoleamine has been shown to suppress not only basal but also TSH-stimulated mitotic activity. The results are in agreement with the hypothesis of a pineal-thyroid negative feedback, assuming the direct inhibitory effect of melatonin on the thyroid growth.